The present invention relates to a developing method and an apparatus therefor which is incorporated in an electrophotographic copier, printer, facsimile transceiver or similar image forming equipment. More particularly, the present invention is concerned with a method of developing a latent image electrostatically formed on an image carrier in a developing region by a developer carried on a developer carrier which faces the image carrier in the developing region, and an apparatus therefor.
With the above-described kind of image forming equipment, it has been customary to locate a developer carrier carrying a thin layer of developer thereon and an image carrier carrying an electrostatic latent image thereon face-to-face in a developing region. An electric field capable of transferring the developer from the developer carrier to the image carrier is generated in the developing region to thereby develop the latent image on the image carrier. This type of developing method has a threshold regarding the transfer of the developer from the developer carrier to the image carrier. The problem is, therefore, that although the developer deposits in an image portion having a surface potential higher than the threshold, it scarcely deposits in an image portion whose surface potential is lower than the threshold, resulting in an image whose tones are poor.
The above problem can be eliminated if an alternating electric field of relatively low frequency is generated in the developing region, as proposed in the past (see, for example, Japanese Patent Publication No. 1013/1989). Such a scheme, however, has a disadvantage that conditioning the alternating electric field for the enhancement of tones causes the image density to decrease while conditioning it for the enhancement of image density causes line images to become thicker.
Another problem with the above-described type of developing method is that when use is made of a non-magnetic toner as a developer, it changes into a powder cloud when caused to move in a reciprocating motion, noticeably lowering the image density (see, for example, Japanese Patent Publication No. 14706/1990).
Today, the demand for higher image quality is increasing in parallel with the diversification of the output information of images to be formed by image forming equipment. To meet this demand, various kinds of developing methods have recently been developed for improving image density while preserving tones, and preventing line images from thickening to thereby implement high quality images.